Patent Application
20080274008
The present invention relates to a corrosion-resistant steel excellent in toughness of a base metal and a weld portion, and a method of manufacturing the same, and more specifically, a corrosion-resistant steel used in various forms under various corrosive environments, such as various containers, vacuum vessels, low-temperature heat exchangers and bathroom components used under corrosive environment with dewing or under indoor environment; such as bridge, support columns, tunnel reinforcing components, interior and exterior materials for buildings, roof materials and fittings used under aerial corrosive environment; such as various reinforcing structures and support columns used under corrosive environment with concrete; and such as marine vessels, bridges, piles, sheet piles and marine structures used under corrosive environments with seawater.
Steels used under various corrosive environments, such as high-temperature and high-humidity corrosive environment, corrosive environment with dewing, aerial corrosive environment, corrosive environment with city water, corrosive environment with soil, corrosive environment with concrete, and corrosive environment with seawater, are generally provided with some anti-corrosion measures. In recent years, in view of improving reliability, simplifying manufacturing and application processes, achieving maintenance-free, saving resources and the like, there have been increasing trends in using Cr-containing steels and stainless steels, for the purpose of improving corrosion resistance of the steel base. Most of conventional techniques have, however, failed in providing a practical measure from an economical point of view, because improvement in the corrosion resistance has resulted in increase in material cost, and have sometimes resulted in only a limited range of applications due to poor strength when austenitic steels were used.
As seen in the above-described examples, any steels containing certain levels of Cr generally became more likely to cause local corrosion as the corrosive environment became more severe, so that as a countermeasure for this problem, further increase in the concentration of Cr or Mo has been a most general technical means for improving the resistivity against corrosion.
In recent years, there have been proposed steels added with Al besides Cr, aiming at improving the corrosion resistance, or both of the corrosion resistance and workability, as disclosed in Japanese Patent Application Laid-Open Nos. 5-279791, 6-179949, 6-179950, 6-179951, 6-212256, 6-212257, 7-3388 and 11-350082 and the like. These steels may be effective to some degree in terms of improvement in the corrosion resistance or both of the corrosion resistance and the workability, but poor in toughness of the base metal and the heat affected zone (HAZ), and this raises a tough obstacle for the steel to be applied to weld structures.
After considering the above-described situations, the present invention is aimed at providing a low-cost, corrosion-resistant steel showing a large corrosion resistance under various corrosive environments such as corrosive environment with dewing, aerial corrosive environment, corrosive environment with city water, and corrosive environment with seawater, and excellent in the toughness in the heat affected zone (HAZ).
Aiming at achieving the above-described objects, the present inventors made extensive studies from every aspect, in order to develop a steel showing excellent corrosion resistance under various corrosive environments such as corrosive environment with dewing, aerial corrosive environment, corrosive environment with city water, corrosive environment with concrete, and corrosive environment with seawater. First, after extensive investigations into techniques for improving the corrosion resistance under the above-described various environments, as well as the toughness of the weld portion, the present inventors found that a steel containing 3 to 11% of Cr, added with 0.1 to 2% of Al, showed a very excellent corrosion resistance under the above-described various corrosive environments. However, this sort of steel typically produces coarse ferrite when heated at 1200° C. or above during welding, due to its wide range of ferrite phase transformation, so that the toughness may degrade to a considerable degree, and may cause cracks and the like after welding. The present inventors then further went through a series of experiments, and found out that a mode of generation of the coarse ferrite phase transformation during welding can be estimated based on a parameter Tp below, expressed using amounts of addition of alloying elements. The parameter Tp can be expressed using concentrations of ferrite-forming elements (Cr, Al) and austenite-forming elements (Mn, Ni, for example) which suppress production of ferrite phase. The present inventors found out that production of ferrite at higher temperatures can be suppressed, when the parameter Tp has a value of not smaller than a predetermined level.
On the other hand, addition of some austenite-forming elements described in the above can suppress production of the coarse ferrite phase in the weld portion, but addition of large amounts of the alloying elements may promote formation of a low-temperature-transformation-forming phase with poor toughness in the process of cooling after rolling of the base metal, and thereby tends to lower the toughness of the base metal. The present inventors then made extensive studies on preventing such embrittlement, defined a parameter Tc which specifies concentrations of the alloying elements capable of ensuring a desirable level of toughness of the base metal after rolling, and found out that a desirable level of toughness can be ensured when the parameter Tc has a value of not smaller than a predetermined level.
Basic concepts of the present invention are as follows.
(1) A corrosion-resistant steel excellent in toughness of a base metal and a weld portion, containing, in % by weight:
C: 0.2% or less;
Si: 0.01 to 2.0%;
Mn: 0.1 to 4%;
P: 0.03% or less;
S: 0.01% or less;
Cr: 3 to 11%;
Al: 0.1 to 2%; and
N: 0.02%, and
having values of 1150 or above, and 600 or above respectively for Tp and Tc expressed by the equations below using concentrations of Cr, Al, C, Mn, Cu and Ni respectively given as % Cr, % Al, % C, % Mn, % Cu and % Ni.
Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); and
Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).
(2) The corrosion-resistant steel excellent in toughness of a base metal and a weld portion according to (1), further containing, in % by weight, any one of, or two or more elements selected from the group consisting of:
Cu: 0.1 to 4%;
Ni: 0.1 to 4%;
Mo: 0.01 to 1%;
V: 0.01 to 0.1%;
Nb: 0.005 to 0.050%;
Ti: 0.005 to 0.03%;
Ca: 0.0005 to 0.05%;
Mg: 0.0005 to 0.05%; and
REM: 0.001 to 0.1%.
(3) A method of manufacturing a corrosion-resistant steel excellent in toughness of a base metal and a weld portion, including the steps of:
heating a steel slab;
the steel slab containing, in % by weight:
C: 0.2% or less;
Si: 0.01 to 2.0%;
Mn: 0.1 to 4%;
P: 0.03% or less;
S: 0.01% or less;
Cr: 3 to 11%;
Al: 0.1 to 2%; and
N: 0.02%, and
having values of 1150 or above, and 600 or above respectively for Tp and Tc expressed by the equations below using concentrations of Cr, Al, C, Mn, Cu and Ni respectively given as % Cr, % Al, % C, % Mn, % Cu and % Ni,
forming a steel plate by hot rolling of the steel slab; and
cooling the steel plate by air.
Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni); and
Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni).
(4) The method of manufacturing a corrosion-resistant steel excellent in toughness of a base metal and a weld portion according to (3), wherein the steel slab further contains, in % by weight, any one of, or two or more elements selected from the group consisting of:
Cu: 0.1 to 4%;
Ni: 0.1 to 4%;
Mo: 0.01 to 1%;
V: 0.01 to 0.1%;
Nb: 0.005 to 0.050%;
Ti: 0.005 to 0.03%;
Ca: 0.0005 to 0.05%;
Mg: 0.0005 to 0.05%; and
REM: 0.001 to 0.1%.
(5) The method of manufacturing a corrosion-resistant steel excellent in toughness of a base metal and a weld portion according to (3), further including, after the step of cooling the steel plate by air, tempering the steel plate at a temperature of Ac1 transformation point or below.
(6) The method of manufacturing a corrosion-resistant steel excellent in toughness of a base metal and a weld portion according to (4), further including, after the step of cooling the steel plate by air, tempering the steel plate at a temperature of Ac1 transformation point or below.
Paragraphs below will explain constitutive elements of the inventive corrosion-resistant steel, concentrations thereof and the like.
C: C is an element improving the strength, but addition to an amount exceeding a predetermined level results in degradation of the toughness in the heat affected zone (HAZ). The upper limit of the C concentration is therefore set to 0.2%.
Si: Si is effectively added to a steel containing 2% or more of Cr, as a deoxidizer and a strengthening element, wherein the concentration thereof less than 0.01% results in only an insufficient effect of deoxidization, whereas the concentration exceeding 2.0% not only saturates the effect but also adversely degrades the toughness of the heat affected zone (HAZ). The range of Si concentration is therefore limited from 0.01% and 2.0%, both ends inclusive.
Cr: Cr is added in order to ensure a desirable level of corrosion resistance, similarly to Al, wherein an amount of addition of 3% or more exhibits the effect, whereas the amount of addition exceeding 11% not only increases the cost, but also impairs again the toughness of the heat affected zone (HAZ). The upper limit of the Cr concentration is set to 11%.
Al: Al is an important element, similar to Cr, in view of ensuring a desirable level of corrosion resistance in the present invention, wherein the concentration of Al is necessarily set to 0.1% or more in view of ensuring a desirable level of corrosion resistance. On the other hand, the amount of addition exceeding 2% extremely widens a temperature range causing the ferrite phase transformation. The concentration of Al is therefore limited to 0.1% to 2%, both ends inclusive.
Mn: Mn in the present invention functions mainly as improving the strength and as an austenite-forming element, and is added to suppress generation of coarse ferrite promoted by Cr and Al added in view of improving the corrosion resistance. More specifically, Cr and Al are ferrite-forming elements as well-known, wherein large amounts of addition of these elements may give a ferrite single phase structure over a range from solidification point to room temperature, without causing transformation, and may considerably degrade the toughness not only in the base metal, but also in the heat affected zone (HAZ). The present inventors made systematic experiments aiming at improving the toughness of the heat affected zone (HAZ) without causing the corrosion resistance, and found out that addition of Mn can avoid the problem. Specific conditions for limitation therefor will be described later, wherein Mn is necessarily added to as much as 0.1% or more, but the amount of addition exceeding 4% enhances the hardening property, so that the addition is limited up to 4%.
N: The less N is contained, the more preferable, because a large amount of addition thereof to steel plate may lower the toughness of the base metal and the heat affected zone (HAZ), so that the upper limit of concentration thereof is set to 0.02%.
P: The less P is contained, the more preferable, because abundance thereof lowers the toughness, so that the upper limit of concentration thereof is set to 0.03%. The concentration thereof ascribable to inevitable contamination is preferably minimized as possible.
S: The less S is contained, the more preferable, too, because abundance thereof lowers pitting resistance, so that the upper limit of concentration thereof is set to 0.01%. Similarly to P, also the concentration of S ascribable to inevitable contamination is preferably minimized as possible.
The present invention further allows addition of the elements below.
Cu, Ni: Both of Cu and Ni exhibit effects of improving the strength, and of suppressing the ferrite generation. In particular, Ni has an effect of improving the toughness of the base metal and the heat affected zone (HAZ). Addition to as much as 0.1% or more is necessary for both of Cu and Ni in order to obtain these effects, wherein the amounts of addition of the both exceeding 4% enhances the hardenability and causes embrittlement. Both of the concentrations of Cu and Ni are therefore set to 0.1 to 4%.
Mo: Mo added to as much as 0.01% or more to a steel added with Cr and Al exhibits an effect of suppressing generation and growth of pitting, without impairing the toughness of the base metal, whereas an amount of addition exceeding 1.0% not only saturates the effect but also degrades the toughness. The concentration of Mo is therefore set to 0.01% to 1.0%.
Nb: Nb is an element improving the strength and toughness without impairing the corrosion resistance, wherein the effect thereof is recognizable at a concentration of as small as 0.005%, whereas the concentration exceeding 0.05% considerably degrades the toughness of the heat affected zone (HAZ). The concentration of Nb is therefore set to 0.005% to 0.05%.
V: V is an element improving the strength without impairing the corrosion resistance, similar to Nb, wherein the effect thereof is recognizable at a concentration of as small as 0.01% or more, whereas a large amount of addition degrades the toughness as well-known. The upper limit of the V concentration is set to 0.1%.
Ti: Ti is an element contributive to refinement of crystal grains at high temperatures through production of nitride, and can particularly improve the toughness of the heat affected zone (HAZ), without impairing the corrosion resistance. Both of refinement of the crystal grains and improvement in the toughness are recognizable at a concentration of as small as 0.005% or more, whereas addition to as much as exceeding 0.03% adversely degrades the toughness of the base metal and the heat affected zone (HAZ), due to deposition of a large amount of carbide. The range of concentration is therefore set to 0.005% to 0.03%.
Ca, Mg: Ca and Mg are elements capable of improving the corrosion resistance in a steel containing Cr and Al. Although much of the mechanism thereof remain unclear at present, it has been made clear that improvement in the corrosion resistance is recognizable at a concentration of as small as 5 ppm or more for the both, whereas the amount of addition exceeding 500 ppm not only saturates the effect of improving the corrosion resistance, but also tends to degrade the toughness. The concentrations of these elements are therefore set to 5 ppm to 500 ppm, both ends inclusive.
REM: In the present invention, also appropriate addition of rare earth metals (REM) can improve the toughness of the base metal and the weld portion, without impairing the corrosion resistance. An amount of addition of 0.001% or more is necessary, whereas a large amount of addition degrades the toughness, so that the upper limit thereof is set to 0.1%.
In the present invention, the parameter Tp expressed by the equation (1) is introduced, in order to improve the toughness of the weld portion, as one major object of the present invention.
Equation (1)
Tp=1601−(34% Cr+287% Al)+(500% C+33% Mn+60% Cu+107% Ni)
where, % Cr, % Al, % C, % Mn, % Cu and % Ni are concentrations of Cr, Al, C, Mn, Cu and Ni (% by weight), respectively.
The present inventors further investigated into relations between concentrations of the alloying elements and the toughness, for the purpose of ensuring a desirable level of toughness of the base metal, and found out that the toughness of the base metal can be evaluated based on the parameter Tc expressed by the equation (2).
Equation (2)
Tc=910+80% Al−(300% C+80% Mn+15% Cr+55% Ni)
As for corrosion-resistant steel of the present invention, the steel slab can be made by the ingot making/breaking down method, continuous casting method and the like. The steel slab may further be processed to give a steel plate by hot rolling, hot forging or the like, or may be hot-worked to give an arbitrary geometry corresponding to a user's need, such as steel pipes represented by seamless steel pipe, shape steels and the like. The hot working can be followed by air-cooling, for example. Tempering at a temperature not higher than Ac1 transformation point, aiming at further improving the strength, will never interfere the effects of the present invention.
The corrosion-resistant steel according to the present invention is applicable, for example, to various corrosive environments, such as high-temperature and high-humidity corrosive environment, corrosive environment with dewing, aerial corrosive environment, corrosive environment with city water, corrosive environment with soil, corrosive environment with concrete, corrosive environment with seawater, and corrosive environments based on any combinations of them.
Each of steels having compositions listed in Table 1 was melted and cast, hot rolled to give a 15-mm-thick steel plate, wherein some of them were further tempered, and subjected to the tests described below.
All test pieces were collected from the center-thickness portion of the plate in the longitudinal direction.
Evaluation of Toughness of Base Metal: Evaluation was carried out based on absorbed energy observed in the Charpy test at −5° C.
Evaluation of Toughness of Heat Affected Zone (HAZ): Impact test of the heat affected zone (HAZ) after being subjected to the welding heat cycles was carried out. The maximum heating temperature and the cooling rate in the test were set to 1400° C. and 15° C./s, respectively. The base metal was also subjected to the impact test. Transition temperatures were determined for the both, and ΔvTrs=([transition temperature of base metal]−[transition temperature after heat cycles]) was determined.
5-mm-thick corrosion test pieces were collected by cutting from a test steel plate, wherein some of them were provided with Zn-base coating (coating thickness: 15 to 25 μm), and then subjected to the test under conditions described below.
Indoor Environment: The uncoated pieces were subjected to a one-hundred-day exposure test in a room with an air conditioner.
Humid Environment: The test pieces were kept at −20° C. for 2 hours, and then kept in an environment of 95% humidity at 25° C. for 4 hours, and this cycle was repeated 13000 times. Size of rust spot was scored for all samples.
Salt Damage Environment: The test pieces were exposed to a coastal splash zone for 17 months.
Results of these tests are shown in Table 2. All of steels marked with A to K are those within the scope of the present invention, and every one of them showed a toughness of the base metal of 100 J or above, and a toughness of the heat affected zone (HAZ), evaluated in terms of ΔvTrs, of −15° C. or above, proving only a small lowering in the toughness. As for corrosion resistance, only a slight rusting of as small as 2 mm or less was observed on some of the pieces, and all pieces showed desirable characteristics.
On the contrary, all of the steels marked with L to U are those according to comparative examples out of scope of the present invention. More specifically, steels L, M and N, having the concentrations of C, Si and Mn, respectively, exceeding the upper limits specified by the present invention, showed almost desirable corrosion resistance, but showed considerable degradation in the toughness. The steel marked with L showed a toughness (ΔvTrs) of the heat affected zone (HAZ) of −40° C., indicating a large decrease. The steels marked with O and P, having amounts of addition of Cr and Al, which are elements contributive to improvement in the corrosion resistance, fallen below the lower limits, showed considerable decrease in the corrosion resistance. The steel marked with Q, having the Al concentration exceeding the upper limit, showed a desirable corrosion resistance, but was degraded in the toughness of the base metal. The steel marked with R, having Ni added as exceeding the upper limit, again showed a desirable corrosion resistance, but was poor in the toughness of the base metal. All of the steels marked with S, T and U have the concentrations of the individual element fallen within the ranges of the present invention, but have value(s) of the parameter(s) Tp and/or Tc out of the ranges of the present invention. More specifically, the steel marked with S is an example having only the parameter Tp fallen out of the range of the present invention, showing a degraded toughness of the heat affected zone (HAZ) of −55° C. The steel marked with T is an example having only the parameter Tc fallen out of the range of the present invention, showing a degraded toughness of the base metal of 83 J. The last steel marked with U is an example having both of the parameters Tp and Tc fallen out of the ranges of the present invention, showing degradation both in the toughness of the base metal and the heat affected zone (HAZ).
The present invention can provide, at low costs, a steel excellent not only in the corrosion resistance in corrosive environment with dewing, and in other various corrosive environments such as indoor environment, aerial corrosive environment and corrosive environment with seawater, but also in the toughness of the heat affected zone (HAZ) which is important for weld structures, and can make a huge contribution to industrial development.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-072438 | Mar 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/06663 | 5/12/2004 | WO | 00 | 9/14/2006 |
